This invention relates to a thin-layers superconductive inductive component, in particular having characteristics of tunable or adjustable inductance. It also relates to a method for producing such components, as well as devices including such components.
This invention belongs to the field of electric and electronic superconductive components for the electrotechnical or electronics sectors, the telephony sector, the antennae and high-frequency passive device sectors, in particular for medical imaging as well as radars and defence electronics.
Thin-layers superconductive inductive components are generally produced by depositing a superconductive film, generally by vacuum methods such as cathode sputtering or pulsed laser ablation, then the definition by lithographic photography of one or more turns. In this technique the size of the device increases with the value of its inductance.
A practical embodiment consists of a coil comprising 5 turns, the external diameter of which is 15 mm, with tracks of 0.4 mm in width at intervals of 0.3 mm having an inductance of 2.12 μH, which is described in the thesis memorandum proposed by Jean-Christophe Ginefri on 16 Dec. 1999 at the Université de Paris XI and entitled <<Antenne de surface superconductrice miniature pour l'imagerie RMN à 1.5 Tesla>>
The technique described above has two main drawbacks:                the surface occupied by each inductive component is significant. For example, the component described in the preceding paragraph occupies a surface of more than 700 mm2:        if the component is integrated into a circuit, it is often necessary to connect the end of the inner turn to a superconductive line. This involves a complex method comprising after the depositing and the etching of the turns:                    a) the depositing and etching of an insulating film,            b) the depositing and etching on this insulator of a second superconductive film having properties similar to those of the first film. This last step is particularly delicate as it is necessary to produce an epitaxial regrowth, a technique which is difficult to control. Other methods enabling the depositing of a coil in thin layers exist, but they present production problems identical to those described here.                        
Moreover, a certain number of methods are known for obtaining inductive components the inductance characteristics of which are easily adjustable, during the production or once implanted in a circuit or an electric or electronic device.
Such an adjustment can be useful in the production stage, for example in order to produce, at low cost, an extensive and homogeneous range of components with different inductances, by changing only a few parameters in the production process.
It is also very useful to have inductive components, the inductance of which can be adjusted subsequently, for example in order to carry out an adjustment or a calibration or a measurement within a device including such components.
The known devices or methods often use an adjustment to the production of the geometric characteristics of macroscopic elements, or a subsequent adjustment of this geometry by a mechanical action. This involves for example adjusting or controlling the position of a ferrite core at the centre of a coil as in the patent U.S. Pat. No. 4,558,295, or of a metal electrode between two dielectric parts as described in the patent U.S. Pat. No. 6,556,415. It can also involve a shift of contact on a conductive track forming a meander deposited in a thin layer, as taught by the US patent application 2002/0190835.
It is also possible to join by electric or electronic connection a certain number of sub-components of known inductance, as the U.S. Pat. No. 5,872,489 proposes, which has obvious limits, for example in terms of number of values obtained and of complexity of production.
Another method is proposed by the U.S. Pat. No. 5,426,409, which consists in controlling by means of a variable current the degree of magnetic saturation of the core of a coil. When the conditions and the frequencies concerned allow, it is also possible to adjust an inductance by means of frequency variation on a semi-conductor material (MESFET GaAs technology, described in U.S. Pat. No. 6,211,753). This type of solution is not however applicable in every case, and is also cannot always be miniaturized beyond a certain limit.
According to the solutions employed, the components obtained can be subject to wear. Often, they have substantial space requirements. They also have limitations in terms of frequency ranges of and/or useable performance ranges.
In addition to the limitations cited above in terms of miniaturization and inductance performance, producing components with varying inductances or adjusting the inductance value of a component therefore presents substantial difficulties.